The invention relates to load-bearing structures and, more particularly, to structures for use in aircraft such as pressure bulkheads, passenger doors, cargo doors, landing gear doors, and the like.
Primary load-bearing structures in aircraft are typically fabricated from a number of discrete parts that are joined together, as by welding, riveting, or other processes, to form the desired structure. The transfer of loads from one part to adjoining parts within the assembly is determined by the joining techniques used, and considerable effort is often invested in analyzing such load transfers and designing the structure to optimize the load transfers. In particular, it is often desirable in the design of aircraft structures to provide a xe2x80x9cfail safexe2x80x9d structure whereby if the primary load path fails because of failure of a part or of a juncture between two parts along the primary load path, there is at least one alternate load path capable of safely providing the requisite load limit capability of the structure. Fail safety is one means of demonstrating compliance with airworthiness standards per Federal Aviation Regulations section 25.571.
The cost of fabricating a load-bearing structure can potentially be reduced by employing monolithic fabricating techniques such that discrete parts are consolidated into a single integral structure, thereby eliminating the necessity of fabricating multiple parts and joining the parts together. Typically, when monolithic structures are employed, however, no effort is made to provide multiple or alternate load paths.
The present invention provides a load-bearing structure employing unique monolithic construction such that multiple load paths exist. The invention is particularly suitable when applied to the manufacture of aircraft structures such as pressure bulkheads, passenger doors, cargo doors, landing gear doors, and the like.
In accordance with a preferred embodiment of the invention, a load-bearing structure adapted to serve as a portion of a boundary enclosing a pressurized or evacuated space is provided having redundant load paths. The load-bearing structure comprises a first web, a first outer attachment structure attached to the outer periphery of the first web, a redundant second web spaced from the first web with inner surfaces of the webs facing each other, a second outer attachment structure attached to the outer periphery of the second web, and stiffeners disposed between the first and second webs and attached to the inner surfaces thereof. The stiffeners are fabricated with at least one of the webs and the attachment structure associated therewith as a monolithic part. The first and second outer attachment structures are adapted to affix the load-bearing structure to a further structure of the boundary enclosing the pressurized or evacuated space. Accordingly, the load-bearing structure defines multiple load paths from the webs to the further structure.
In accordance with a further preferred embodiment of the invention, the load-bearing structure for a pressurized space can prevent sudden depressurization of the space and consequent xe2x80x9cblow outxe2x80x9d of the structure in the event of failure of the web exposed to the pressurized fluid. To this end, the web not exposed to the pressure defines at least one aperture therethrough such that if the web exposed to the pressure fails, fluid pressure is relieved through the aperture so as to limit the rate of pressure relief.
The invention further provides such a load-bearing structure in the form of a pressure bulkhead for a pressurized compartment of an aircraft, the outer attachment structures being adapted to attach the bulkhead to fuselage structure of the aircraft. The bulkhead preferably includes intercostal members integrally fabricated with and projecting generally axially from one of the webs and corresponding outer attachment structure, the intercostal members serving to attach the bulkhead to the fuselage structure of the aircraft. In one embodiment, the bulkhead is adapted to be attached to fuselage structure with the first web facing into the pressurized compartment and the second web facing outward therefrom, and the second web defines the aperture for controlled pressure relief. Alternatively, the aperture can be defined in the first web. The web facing into the pressurized compartment preferably includes beams attached to the outer surface thereof for providing sites for attachment of aircraft systems and/or other structural items such as floor beams or brackets.
Advantageously, the stiffeners located between the webs include radial stiffeners that radiate outward from central regions of the webs toward the outer peripheries of the webs. The stiffeners preferably further include circumferential stiffeners that extend generally circumferentially between the radial stiffeners and collectively form at least one ring encircling the central regions of the webs.
The load-bearing structure of the invention can also take other forms. For example, in one embodiment, the structure is adapted to serve as a door for an aircraft, the outer attachment structures being adapted to attach the door to a door frame of the aircraft. Where the door is to be used in a pressurized compartment, one of the webs comprises an outer pressure web for supporting fluid pressure loading and the other web comprises an inner web and defines at least one aperture therethrough. Alternatively, the door can be fabricated as a non-pressurized door. As another example, the structure can be fabricated as a door surround structure for an aircraft, one of the webs comprising an inner web for facing into the aircraft and the other web comprising an outer web for facing out from the aircraft, each web defining a doorway opening bounded by an inner periphery of the web, with one or more apertures being defined in the inner web.